Polyethylene is typically manufactured according to the Phillips process in a slurry system. In this system high purity ethylene is fed to a loop reactor. A low boiling point hydrocarbon such as isobutane is used to dissolve the ethylene monomer and to suspend catalyst and polymer particles within the reactor. Ethylene gas, the dilluent and powdered catalyst are fed continuously into the loop reactor in which the contents are rapidly circulated by means of a pump. The reactor is typically maintained at a temperature of the order of 100° C. and a pressure of the order of 4 MPa. As the process continues, polymer particles start to form and the larger ones precipitate and enter a settling zone from which concentrated slurry is discharged. The production system is continuous such that the product is removed at the same rate at which it forms.
Because of the high pressure within the reactor it will be appreciated that the discharged slurry contains ethylene which it is desirable to recycle in order to avoid waste. The polymer must also be separated from other materials. The slurry is therefore supplied from the loop reactor to a so-called flash tank or flash drum which is maintained at a much lower pressure than the reactor. This results in the ethylene and low boiling point dilluent flash-vaporising overhead. The monomer gas and the dilluent is then transferred to a compressor and reused.
However, it is necessary to purify this “flash gas”, and in particular to remove small particles known as “fines” therefrom as these could damage the compressor. For this reason the vapour is passed firstly to a cyclone and from there to a bag filter. Both the cyclone and the bag filter are connected to collection tanks known as “drop out pots” which collect solid matter. This matter is then fed via rotating valves (which prevent gas flowing therethrough) to a dryer called a “fluff dryer” along with the slurry from the flash tank. The purified gas is then fed via a so-called guard filter to the flash gas compressor. The guard filter is intended merely to protect the compressor in the event of a failure elsewhere in the system.
The fluff dryer dries the matter fed to it to leave polymer powder. Additives are added if required and the powder is then sent on to an extruder in which it is melted and formed into pellets.
The system described above is satisfactory in most circumstances. Over a period of time, the filters, which are typically bag filters, become blocked and the process must be shut down so that these may be cleaned or replaced. Normally, this is not a serious problem because this maintenance is only required infrequently.
However, it has been found that in certain circumstances the filter and sometimes the cyclone clog up after only a short period of time. It has been found that this occurs when polymer having a high comonomer (such as 1-hexene and 1-octene) content is being produced. Hexene may be added to the olefin feed in order to control density of the polymer and it is believed that condensed hexene and oligomers make the overall product sticky. This in turn causes rapid clogging of the cyclone and bag filter. Comonomer 1-hexane also wets the filters which increases this clogging. It is thought that the higher hexene content results in lower temperatures which in turn create an increased amount of condensation which clogs the filter and cyclone.
As a consequence of this, it is only possible to have comparatively short production runs of high hexene content polymers with a practical limit of around 2000 tons. After this, low hexene content products have to be produced in order to clean the cyclone and bag filter. It will be appreciated that this is a serious problem if large volumes of high hexene content product are to be produced.